Odor sensor with organic transistor circuitry

ABSTRACT

Circuits include at least one-odor sensitive organic transistor having a conduction channel whose conductivity changes in response to certain odors. The organic transistors are interconnected to increase their response to selected odor signals. The organic transistors may be interconnected to form a ring oscillator whose frequency of oscillation changes in response to an odor signal and in which the alternating signal applied to the gate electrodes of the organic transistors enhances their recovery and reduces their drift.

BACKGROUND OF THE INVENTION

[0001] This invention relates to circuitry employing organic transistorsand, in particular, organic field effect transistors (OFETs) to detectchemical odors/vapors/gases (analytes).

[0002] Many different types of OFETs are known. By way of example, FIG.1 shows the structure of an OFET 10 having a semiconductor body region12 with a source electrode 14 and a drain electrode 16 defining the endsof a conduction channel through the semiconductor body 12. The OFET 10also includes an insulator layer 18 and a gate (control) electrode 20 towhich a voltage may be applied to control the conductivity of thesemiconductor body region (i.e., the conduction channel). The OFET ofFIG. 1 is manufactured to have organic material in its semiconductorbody region 12 that can absorb analytes and which, in response to theabsorbed analytes, changes the conductivity characteristics of theconduction channel. As illustrated in FIG. 1, analytes(vapors/odors/gases) may flow over the OFET for a period of time.Ensuing changes in the conductivity of the OFET may be measured as shownin FIG. 1A by sensing the current (I_(d)).

[0003] In known circuitry, the OFETs have been used as discrete devices.As shown in FIG. 1A, the source of an OFET may be connected to a firstpoint of operating potential (e.g., VDD) and its drain may be connectedvia a load resistor RL to a second point of operating potential (e.g.,ground potential). The gate of the OFET may be biased via resistors R1and R2 to produce a desired operating direct current (d.c.) bias levelwithin the source-drain (i.e., conduction) path of the OFET. The OFETmay then be subjected to a flow of analytes which causes itsconductivity to change. The corresponding change in conductivity of theOFET is then detectable by a circuit connected to the drain and/or thesource of the OFET.

[0004] A problem with known OFETs is that their sensitivity to theanalytes is relatively low. Also, known OFETs are subject to drift andthreshold shift as a function of time, as shown in FIG. 2A and FIG. 2B,respectively. In FIGS. 2A and 2B, it is seen that, for a fixed biascondition, source-to-drain current (I_(d)) of an OFET changes (e.g.,decreases) as a function of time. This is the case when there is nosignal input (i.e., no odor), as illustrated by waveform A of FIG. 2Aand waveform portion C in FIG. 2B. This is also the case following theapplication of an odor to the OFET, as illustrated in waveform B of FIG.2A and in waveform portion D in FIG. 2B. That is, for a fixed biascondition, the current through the conduction path of the OFET changes(drifts) as a function of time. OFETs may also be subjected tohysteresis and offsets. As a result of these characteristics, it isdifficult to use OFETs in known discrete circuits to differentiate aninput signal from background conditions and to determine or measure thefull extent of the input signal.

SUMMARY

[0005] Problems associated with the characteristics of OFETs, such astime related drift, detract from their use as sensors and amplifiers oftheir sensed signals when the OFETs are used as discrete devices.Applicants recognized that OFETs should be incorporated in circuitsspecifically designed to overcome and/or cancel the problems associatedwith certain characteristics of OFETs such as their drift, thresholdshift and hysteresis.

[0006] Circuits of various embodiments include at least oneodor-sensitive organic transistor having a conduction channel whoseconductivity changes in response to certain ambient odors.

[0007] In one embodiment, organic transistors are interconnected toincrease their response to selected odor signals and such that therecovery of the organic transistors is enhanced and their drift isreduced. In a particular embodiment, the organic transistors areinterconnected to form a ring oscillator whose frequency of oscillationchanges sharply in response to an odor signal and in which thealternating signal applied to the gate electrodes of the organictransistors enhances their recovery and reduces their drift.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In the accompanying drawing, like reference characters denotelike components; and

[0009]FIG. 1 is a highly simplified cross-section of a known OFETsuitable for use in various embodiments;

[0010]FIG. 1A is a prior art circuit diagram of a discrete OFET circuitto sense and amplify signals resulting from odors;

[0011]FIGS. 2A and 2B are diagrams illustrating the drift in the currentthrough the conduction path of an OFET as a function of time;

[0012]FIGS. 3A and 3B are the symbolic representation of P-type OFETsand P-type FETs, respectively, used in this application;

[0013]FIGS. 4A and 4B are the symbolic representation of N-type OFETsand N-type FETs, respectively, used in this application;

[0014]FIG. 5 is a schematic diagram of an OFET-differential amplifiercombination embodiment the invention;

[0015]FIG. 6 is a schematic diagram of an OFET- amplifier combinationembodiment;

[0016]FIG. 7 is a block diagram of a system employing OFETs in oneembodiment;

[0017]FIG. 8 is a schematic diagram of a ring oscillator circuitemploying OFETs in one embodiment;

[0018]FIG. 9 is a diagram of waveforms associated with one embodiment ofthe circuit of FIG. 8;

[0019]FIG. 10 is a schematic diagram of another ring oscillator circuitemploying OFETs in one embodiment;

[0020]FIGS. 10A and 10B are schematic diagrams of cascaded invertersusing OFETs for analyte sensing;

[0021]FIG. 11 is a block diagram of an odor sensor in one embodiment;

[0022]FIG. 12 is a cross section of an OFET suitable for use in circuitsembodying the invention;

[0023]FIG. 13 is a drawing of various molecular structures of severalmaterials used to make OFETs; and

[0024]FIG. 14 is a drawing of examples of molecular structures of odorsto be sensed by circuits of various embodiments.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] In the discussion to follow, reference is made to organic fieldeffect transistors (OFETs) which may be used to sense odors, vapors,chemicals and/or gases (analytes). These terms are used interchangeablyand are intended to include each other in the specification and in theclaims appended hereto. OFETs have been described in the literature andthe teachings of these references as to the manufacture of thesetransistors and their reported characteristics are incorporated hereinby reference. In the discussion to follow references to OFETs will alsoinclude organic thin film transistors (OTFTs) and any device havingsimilar characteristics.

[0026] To better understand the discussion to follow and the drawingsappended hereto, certain characteristics of OFETs will first bediscussed. OFETs (or OTFTs) may be of N-conductivity type orP-conductivity type. To more easily differentiate the OFETs from knownfield effect transistors (FETs), the following symbology will be used inthe appended drawings: (a) P-type OFETs will be shown as illustrated inFIG. 3A; (b) P-type FETs will be shown as illustrated in FIG. 3B; (c)N-type OFETs will be identified as shown in FIG. 4A; and (d) N-type FETswill be identified as shown in FIG. 4B. The drawings for the OFETsinclude a rectangular box, with an X through the box, indicative of thesemiconductor body with source and drain electrodes attached to thesemiconductor body representing the ends of a conduction path (orchannel) through the semiconductor body. The semiconductor body iselectrically insulated from a gate (control) electrode, g, to which abias voltage or a signal may be applied to control the conductivity ofthe semiconductor body. A P-type OFET is shown with an arrow pointingtowards the body of the OFET and an N-type OFET is shown with an arrowpointing away from the body of the OFET. The semiconductor body definesthe conduction path and the OFET includes source and drain electrodesdefining the ends of the conduction path. OFETs, like FETs, aregenerally bi-directional conducting devices. Therefore: (a) the sourceof a P-type OFET or of a P-FET is defined as the one of the twoelectrodes connected to the semiconductor body whose potential is morepositive than the other (drain) electrode; and (b) the source of anN-type OFET or of an N-FET is defined as the one of the two electrodesconnected to the semiconductor body whose potential is less positivethan the other (drain) electrode.

[0027] As noted above, OFETs are typically subject to several problems:

[0028] a) low sensitivity; (b) drift; (c) hysteresis; and (d) variationof their threshold voltage (V_(T)) as a function of time.

[0029] The various embodiments are directed to circuits that use OFETsbut are configured to reduce or compensate for the problems associatedwith low sensitivity, drift, threshold shift and hysteresis.

[0030]FIG. 5 illustrates the use of an organic transistor, OFET M2, as asensor of odors and as an amplifier of the signal sensed. In thediscussion to follow, when an odor flows over an OFET and the OFET isturned-on and/or biased into conduction, the OFET may be said to be“sensing” or “sniffing” the odor and to be in a sensing or sniffingmode. OFET M2 forms part of the input stage of a differential amplifier.The amplifier includes selectively enabled adaptive feedback circuitry,which enables the background, drift and threshold shift conditions to beeffectively subtracted from the amplified output signal. When theamplifier circuit is in a standby non-sensing mode (i.e., not sensing orsniffing any odors, gases, chemical vapors etc.) negative feedback isused to cancel any drift or threshold shift due to OFET M2. When thecircuit is in a sensing mode (i.e., “sniffing”) the feedback loop isopened with the circuit biased in a high gain state so it can respondquickly and with high gain to an input signal.

[0031] In the circuit of FIG. 5, transistor M2 is an odor-sensitiveP-type OFET. For ease of illustration the other transistors used in thecircuit are non-odor sensitive FETs or OFETs. Transistors M1, M2, and M5form the input stage of a differential amplifier, with the sourceelectrodes of M1 and M2 being connected to the drain of M5. Thus, M1 andM2 compete for the current from current-source M5. The source electrodeof M5 is connected to a point of fixed operating potential (i.e., VDD)and a bias voltage V_(B) is applied to the gate of M5 causing a constantcurrent, Io, to flow through the conduction path of M5. The current Iois equal to the sum of the current I1 through the conduction path of M1and the current I2 through the conduction path of M2; that is, Io=I1+I2.The value of the current flowing through the conduction paths of M1 andM2 is a function of their relative gate voltages. The lower the gatevoltage of M1 with respect to M2, the greater is the fraction of thecurrent Io that flows through it; similarly for M2. The drain of M1 isconnected to node A1 and the drain of M2 is connected to node A2.

[0032] The current I1 flowing into node A1 is mirrored via a currentmirror circuit comprised of transistors M3 and M6 to produce a currentI6 flowing through the conduction paths of transistors M6 and M8. Forease of illustration, assume that the current current I6 which is equalto the current I8 is also equal to the current I1 ; i.e., I6=I8=I1. Thecurrent through M8 is then mirrored via a current mirror comprised oftransistors M8 and M9 to cause a current I9 to flow through theconduction path of M9 into node A3. The sources of M8 and M9 areconnected to VDD volts and their gates are connected in common to thedrains of M8 and M6. When M8 and M9 have similar geometries, their draincurrents will be substantially equal for the same gate-source biasconditions. In that case, the current I9 flowing into node A3 is equalto I6 which is, in turn, equal to I1; i.e., I1=I6=I9.

[0033] The drain current I2 through M2 flows into node A2 and ismirrored via the current mirror transistor combination of M4 and M7. Thesources of M4 and M7 are connected to ground potential and the gate ofM7 is connected to the gate and drain of M4. When M4 and M7 are ofsimilar geometries their drain currents will be substantially the samefor like gate-source bias conditions. Thus, M4 and M7 function as acurrent mirror to produce a current I7 through the conduction path of M7which is drawn out of node A3. In that case, the current I7 is equal tothe current I2. For the above conditions, if I1 is equal to I2, thecurrent I9 flowing into node A3 is equal to the current I7 flowing outof node A3. Where M9 and M7 have essentially equal impedances for thecorresponding bias condition, the voltage at node A3 will besubstantially equal to VDD/2 when I1 is equal to I2. It should also beappreciated that, since M7 and M9 are effectively high impedance currentsources, a small difference between the currents I9 and I7 results in alarge voltage differential at node A3. Thus, the circuit has a verylarge open circuit (i.e., when there is no feedback) voltage gain.

[0034] Thus, the pair of currents I1 and I2 are mirrored via the currentmirrors formed by M3-M6, M4-M7, and M8-M9 and compared with each otherto generate an output voltage at node A3. If the current through M2exceeds that through M1, then the node voltage A3 is driven to a valuenear ground. If the current through M1 exceeds that through M2, then thenode voltage A3 is driven to a value near VDD volts. Thus, transistorsM1-M9 implement a wide-output range differential amplifier with inputsgiven by the gate voltages of M1 and M2 and an output given by thevoltage at node A3.

[0035] The gate of OFET M2 is connected to a relatively constant biasvoltage source V2. To better illustrate the operation of the circuit andthe role of M2 as a sensor, an input signal source 71 is shown connectedbetween the source and gate of M2. The source 71 and its connections areshown with dashed lines since this source is internal to OFET M2. TheVin source 71 depicts the equivalent input signal due to a mobility orthreshold voltage change in transistor M2 when an odor is “puffed” onto(i.e., applied to) it. An odor “puffed” onto transistor M2 is equivalentto the application of an input signal to its gate. When an odor to besensed is applied to M2, the feedback loop is opened (i.e., P-typetransistor M12 is turned off by driving voltage V12 to an active highstate). Transistor M2 is the only odor sensitive transistor in thecircuit of FIG. 5.

[0036] The gate of transistor M1 is connected to the output (i.e., nodeA5) of a low pass filter whose input is connected to the output (i.e.,node A3) of the differential amplifier to provide a negative feedbackconfiguration. That is, output node A3 of the amplifier is applied tothe input of a source follower stage (i.e., the gate of M10) comprisedof transistors M10 and M11. The source of M11 is at VDD volts and a biasvoltage V11 is applied to the gate of M11 to establish a current throughM11 and M10. The source of M10 is connected to the drain of M11 at anoutput node A4 and the drain of M10 is returned to ground potential. Theconduction path of a transistor switch M12 is connected between node A4and node A5 which is connected to the gate of M1. A capacitor C1 isconnected between the gate of M1 and ground potential. The low passfilter is implemented with transistors M10, M11 and capacitor C1. Whentransistor M12 is turned on closing the feedback loop (i.e. the circuitis not “sniffing”), the output at node A4 of the source follower isconnected to capacitor C1 (via the low “ON” impedance of M12) such thattransistors M10, M11 and capacitor C1 implement a weakly nonlinear lowpass filter. The time constant of the low pass filter may be controlledby altering V11 or the value of the capacitance of C1 or both.

[0037] When the feedback loop is closed (i.e., M12 is turned on), anydrift or change in the conductivity of M12 is effectively cancelledbecause a conductivity change of M2 causes a corresponding change in I2.The change in I2 then causes a corresponding change in the voltages atnodes A3 and A4. The change at A4 is then applied via M12 to the gate ofM1 with a magnitude and polarity to cause a change in I1 which cancelsor offsets the change in I2 caused by M2 (i.e., negative feedback tendsto cause I1 to equal I2). Thus, when the negative feedback loop isclosed, the gate voltage of M1 adapts to compensate (or cancel) forlong-term, time dependent, changes between the threshold or mobility oftransistors M1 and M2 and automatically keeps the differential amplifieroutput at its balanced midpoint.

[0038] As noted above, when the circuit is not sniffing, the negativefeedback is turned on and causes the circuit to adapt and compensate forany long-term differences between the transistor characteristics of M1and M2. The negative feedback is turned on by causing V12 to go low(e.g., 0 volts) and transistor M12 to be turned on. When M12 is turnedon, the feedback voltage applied to the gate of M1 causes the currentsI1 and I2 to be substantially equal. Assume that M3, M4, M6 and M7 areall made to the have the same geometry. Then, for I1 equal to I2, thecurrent I2 is mirrored through M7 so a current equal to I2 is drawn fromnode A3. Concurrently, the current I1 is mirrored through M6 and M8 andthen mirrored via M9 to produce a current equal to I1 flowing into nodeA3. For I1 equal to I2, the voltage at node A3 will be equal toapproximately VDD/2. The voltage at node A3 is applied to the input ofsource follower stage M10 to produce a similar output at node A4. WhenM12 is turned on, the voltage at A4 is applied, via the conduction pathof M12, to the gate of M1 and will tend to cause equal currents to flowthrough M1 and M2. The high degree of feedback when M12 is turned onensures that any drift in M2 gets compensated (i.e., the effect of thedrift is effectively cancelled). Thus, when the circuit is ready to beused to sense (“sniff”) the presence of any vapors or chemicals,transistor M12 is turned off by the application of a high voltage (e.g.,VDD) to the gate of M12. When that occurs, the circuit is biased at anoptimal operating point to respond to signals resulting from odors being“puffed” (applied) to the sensing OFET M2.

[0039] When the circuit is about to sniff or is sniffing, the feedbackloop is opened (i.e., M12 is turned off) and the circuit sits at itshigh-gain balanced midpoint, ready to amplify any odor-caused change inthe current through M2. The voltage at A3 serves as the output of thecircuit with changes in the voltage of A3 reflecting changes induced bythe odor response of M2.

[0040] This is best illustrated as follows. When the circuit is ready tosniff the presence of an odor, the feedback loop is opened (i.e.,transistor M12 is turned off). When the loop is opened, capacitor C1 ischarged (and remains so for some time) to the voltage present at theoutput of the amplifier immediately before M12 was turned off. Thus, thegate voltage of M1 which represents one of the two inputs of thedifferential amplifier is held at a value representative of the gatevoltage just before the feedback loop is opened. When the odor isapplied, M2 responds and its conductivity is modified by the chemicalspresent in the air or vapor being “sniffed”. If the conductivity of M2is decreased by the “input signal” then the current I2 is decreasedrelative to the current I1 and the voltage at node A3 will rise sharplyand quickly in response thereto. On the other hand, if the conductivityof M2 is increased by the “input signal” then the current I2 isincreased relative to the current I1 and the voltage at node A3 willdrop sharply and quickly in response thereto. In either case a goodindication of the input signal condition will be produced at the outputof the amplifier with the d.c. shift and drift substantially removedfrom the output signal.

[0041]FIG. 5 also shows a switching subcircuit formed by transistorsM12, M13, and M14. The implementation of the low pass filter and theswitching subcircuit are now briefly described. Transistor M12implements a switch that is turned on when the voltage V12 is low.Normally, V12 is driven low when the circuit is not “sniffing” and isdriven high when the circuit is “sniffing”. The conduction channels oftransistors M13 and M14 may be ratioed to have half the width (W) andthe same length (L) as transistor M12. They help to alleviate chargeinjection problems caused by the switching voltage of M12. The chargeinjection is alleviated by having a signal complementary to V12 driveM13 and M14. The charge injection is dominated by the overlapcapacitances of transistor M12; the overlap capacitances ofshorted-and-ratioed transistors M13 and M14 match those at the sourceand drain ends of M12 and serve to cancel the effects of positive chargeinjection from M12 with negative charge injection from M13 and M14.

[0042]FIG. 6 shows a circuit in which an odor responsive P-type OFET M21is interconnected with a transistor M11 to form a common-sourceamplifier. The output (node A31) of the common-source amplifier isconnected to the input of a source (voltage) follower stage comprised oftransistors M31 and M41 whose output (node A41) is selectively fed backto the gate of M11 via transistor M121. As in FIG. 5, when odors/vaporsare “puffed” onto OFET M21 the feedback loop is opened, and thecommon-source amplifier amplifies the signal due to the odors/vapors.

[0043] In FIG. 6, the source of M21 is connected to a power terminal 81to which is applied VDD volts and its gate is connected to a constantbias voltage V21. The drain of M21 is connected to the drain of anN-type FET, M11, at output node A31. The source of M11 is connected toground. Node A31 is connected to the gate of source follower transistorM31 whose drain is connected to terminal 81 and whose source isconnected to terminal A41. Transistors M11 and M21 form a common-sourceamplifier with a control input being the gate voltage of M11 and asignal input being the current through M21 responsive to theodors/vapors puffed onto M21. The output of the common-source amplifieris the voltage at node A31. If the current through M21 exceeds thecurrent through M11, then the node voltage A41 is driven near VDD. If,on the other hand, the current through M11 exceeds that through M21, thenode voltage A41 is driven near ground. The output of the common-sourceamplifier is connected to the input of a source follower stage (the gateof M31) whose output (A41 at the source of M31) is fed back to the gateof M11 via switching transistor M121.

[0044] As in FIG. 5, an input signal source 71 a is shown (with dashedlines) connected between the gate and source of M21 to indicate thesignal input function of the sensor, internal to M21, when an odor/vaporis puffed onto M21. That is, signal source 71 a represents the effect ofa mobility or threshold change on and within transistor M21 when an odoris puffed onto it. Typically, the odor is puffed onto transistor M21when the feedback loop is open (i.e., M121 is turned-off). TransistorM21 is the only odor sensitive transistor in the circuit; any otherorganic transistor in the circuit is assumed to be odor insensitive.

[0045] Transistors M31 and M41 form a standard N-type FET sourcefollower stage whose bias current is set by a voltage V41 applied to thegate of M41. Transistor M121 is turned on when the circuit is notsniffing. When M121 is turned on the output of the source follower istied to a capacitor C11 such that transistors M31, M41 and the capacitorC11 implement a weakly nonlinear low pass filter. The time constant ofthe low pass filter may be controlled by altering V41, the capacitanceof C11, or both. Transistor M121 implements a switch that is turned onand off by a signal source 121 producing a voltage V121.

[0046] When the circuit is not sniffing, the source 121 applies a lowvoltage to the gate of M121 to enable the negative feedback loop andcause the voltage at A41 to be applied to capacitor C11 and the gate ofM11. Thus, during the nonsensing mode, the gate of transistor M11 isconnected to a low pass filtered version of the common-sourceamplifier's output in a negative feedback configuration. Consequently,during this mode, the gate voltage of M11 constantly adapts tocompensate for long-term changes in the threshold or mobility oftransistor M21 and keeps the output A31 of common-source amplifier (M11,M21) at its balanced equilibrium.

[0047] When the circuit is sniffing, the negative feedback is turned offand the circuit sits at its gain balanced equilibrium, ready to amplifyany odor-caused change in the current through transistor M21. The outputvoltage at A31 reflects changes induced by the odor response of M21.When the circuit is not sniffing, the negative feedback is turned on andcauses the circuit to adapt and compensate for any long-term changes inthe characteristics of OFET transistor M21.

[0048]FIG. 6 also shows a switching sub-circuit formed by transistorsM121, M61, and M71 which is active only when V121 is active low. Theimplementation of the lowpass filter and the switching subcircuit arenow briefly described. Transistors M61 and M71 are ratioed to have halfthe W and the same L as transistor M121. They help to alleviate chargeinjection problems caused by the switching voltage of M121. The chargeinjection is alleviated by having a signal (V13) complementary to V121drive M61 and M71. The charge injection is dominated by the overlapcapacitances of transistor M121. The overlap capacitances ofshorted-and-ratioed M61 and M71 match those at the source and drain endsof M121 and cancel charge injection from M121 with charge injection fromM61 and M71.

[0049] Features of the circuits of various embodiments, which werediscussed above in FIGS. 5 and 6, are shown in FIG. 7. FIG. 7 includes ahigh gain amplifier 91 responsive to signals from an OFET sensor that isintegral to one of the amplifying devices in amplifier 91. The output ofthe amplifier is selectively fed back by means of a switching network 92and a low pass filter 93 to a control input of the amplifier 91. Theswitching network is turned on and off as a function of a sniff signalcircuit 94 which controls the application of chemical odors/vapors/gases(analytes) to the sensor contained within the high gain amplifier.During a non-odor-sensing period of time, the switching network 92closes the negative feedback loop such that the low pass filter 93 iscoupled between the output of amplifier 91 and an input of amplifier 91.During an odor-sensing period of time, the switching network is open sothat the feed back loop is removed from the circuit, and the high gainamplifier 91 amplifies signals resulting from the flow of odors over theOFET sensor.

[0050] To better explain other embodiments, assume, as shown in FIGS. 1Aand 2B, that a discrete OFET is biased to conduct a source-drain currenthaving a value I_(D1) prior to any odor being applied to the OFET, Whenan odor is “puffed” onto the OFET, the source-drain current changes fromI_(D1) to I_(D2) in response to an odor (analyte) incident on the OFETfrom a time I1 to a time I2. Thus, after an odor signal is applied to anOFET, the source-to-drain current changes. The change in thesource-to-drain current persists even after the removal of the odor.When an odor is applied for a given time (e.g., 5 seconds), it normallytakes a much longer period of time (e.g., one minute) for the OFETcurrent to recover from the value of I_(D2) to a value approximatelyequal to that of I_(D1)

[0051] After an OFET is subjected to an odor signal, applying anelectrical bias cycle to the gate of an OFET facilitates its recovery tothe condition existing prior to the application of the odor. That is, byapplying an electrical signal to the gate of the OFET which goespositive and negative (or negative and positive) relative to the source(and/or drain) of the OFET, the OFET recovers more quickly and thedegree of recovery is enhanced. Full recovery means the return of thedrain current of the OFET to the level it would have had had an odorsignal not been applied to the OFET.

[0052] In various embodiments, OFETs are operated so that they returnmore quickly to the existing operating condition extent immediatelybefore the application of the selected odor (analyte) to the circuit.Ring oscillators employing OFETs to sense the presence of odors are veryuseful as sensing circuits for selected odors.

[0053] Ring Oscillator Sensors

[0054] Using OFETs in a ring oscillator circuit eliminates many of theabove-discussed problems associated with OFETs. Referring to FIG. 8there is shown five complementary inverters (I1-I5) interconnected toform a ring oscillator. In FIG. 8 each inverter (I1-I5) includes aP-type OFET (P1-P5) and an N-type FET (N1-N5). In one embodiment of theFIG. 8 circuit, the P-type OFETs were made of didodecyl α-sexithiophene(DDα6T) material and the N-type FETs were made of hexadecafluoro copperphthallocyanine (F₁₆CuPc) material. In that embodiment, the materialDDα6T was used to form the P-type OFETs because DDα6T is sensitive tothe analyte octanol and esters such as allyl propionate which are theanalytes selected to be sensed by the circuit. In contrast, the materialF₁₆CuPc was used to make the N-type OFETs, because it is insensitive tooctanol and esters such as allyl propionate. Therefore, the N-type OFETs(N1-N5) are insensitive or, at least, less sensitive to the selectedanalytes and could be replaced by standard N-type FETs.

[0055] The source electrodes of the P-type transistors (P1-P5) areconnected to a power terminal 81 to which is applied +VDD volts. Thesource electrodes of the N-type FETS (N1-N5) are connected to a powerterminal 83 to which is applied ground potential. The gate electrodes ofthe two transistors forming each inverter are connected in common anddefine a signal input terminal to the inverter. The drain electrodes ofthe two transistors forming each inverter are connected in common anddefine a signal output terminal of the inverter. Starting with the firstinverter, the output of each inverter along the chain is connected tothe input of the next inverter along the chain, except for the output ofthe last inverter (e.g., I5) which is fed back to the input of the firstinverter. Note that there is some capacitance, C, (which may beparasitic or discrete) associated with the input (gates) of eachinverter. The combination of the effective output impedance of eachinverter and the input capacitance of the next stage determines the timeconstants of each stage and the frequency of oscillation of the circuit.

[0056] In one embodiment, the oscillation frequency of the 5-stage allorganic F₁₆CuPc/DDα6T complementary ring oscillators ranged from a fewHz to several kHz. A selected analyte was “puffed” onto the ringoscillator circuit. The analyte reduced the conductivity of the P-typeOFETs. In the discussion to follow, it is assumed that the conductivityof the OFET decreases when subjected to a gas (analyte). However, itshould be understood that other OFETs have conductivities that increasewhen subjected to an odor (analyte). For OFETs whose conductivityincreases in response to the presence of an odor, the circuitconfigurations discussed are also suitable. However, the response of thecircuit would be the inverse of that described below (i.e., thefrequency of oscillation would increase rather than decrease).

[0057] The mobility of the discrete F₁₆CuPc transistors and of the DDα6Ttransistors, measured on devices fabricated alongside the ringoscillators, was approximately equal to ˜10⁻² cm²/V-s. The response ofthe circuit was measured with an oscilloscope with a high inputimpedance (50 M ohm) probe. The response of a particular circuitconfigured as illustrated in FIG. 8 is shown in FIG. 9. The change infrequency as a consequence of the odor is clearly seen. Due to thedecrease in the conductivity of the P-type OFETs, there is an increasein the RC time constants of the inverting stages. This causes theoscillation frequency to decrease very sharply. Note that FIG. 9 depictsthe response of the circuit of FIG. 8 to an analyte “puffed” onto thecircuit from time t1 to time t2 (approximately 5 seconds). As a resultof the incident odor “puffed” onto the circuit the frequency changedfrom around 550 Hz to around 280 Hz. Thus a frequency change of nearly50% was observed. A change in the amplitude of the oscillations was alsoobserved (i.e., change in Vmax). The observed change is also muchgreater than the change observed in a discrete OFET in response to thesame odor intensity. This demonstrates that a circuit of the type shownin FIG. 8 is a better odor/gas sensor than using a circuit using asingle OFET.

[0058] Referring to FIG. 9, it is also seen that beginning at time t2,after the odor (analyte) is no longer applied to the circuit, thecircuit begins to return to its condition prior to application of theodor (analyte). Applying an alternating signal to the gate of an OFEThaving a polarity to turn-it-on harder for a first time period and thenhaving a polarity to turn-it-off for a second period of time, tends toenhance the recovery of the OFET to the state it had prior to theapplication of an analyte. This is in sharp contrast to the response ofthe discrete OFET shown in FIG. 2B, where the response of the discreteOFET does not begin to recover immediately after removal of the odor(analyte).

[0059] When the ring oscillator circuit of FIG. 8 is exposed to aselected analyte, the mobility of the material (DDα6T) is changed andthe oscillation frequency changes. This provides a convenient means ofmeasuring the presence and concentration of the analyte. By making theOFETs sensitive to certain particular analytes and not to others it isalso possible to ascertain the presence of these certain analytes.However, usually odorant detection will be done by pattern recognitionbased upon the responses of several sensors. Therefore, it is generallynot necessary to have sensors that respond only to a particular odorant.

[0060] In the circuit of FIG. 8 the P-type transistors P1-P5 are OFETsformed on an integrated circuit (IC) by similar masking and processingsteps. It is possible to obtain a still higher gain response by usingOFETs of complementary conductivity as shown in FIG. 10. FIG. 10 isanother embodiment of an oscillator circuit in which complementaryinverters are arranged such that in every other inverter (e.g., the oddnumbered inverters) the P-type transistor is an odor-sensitive OFET andin the intermediate inverters the N-type transistor is an odor-sensitiveOFET. The OFETs in the circuit of FIG. 10 are formed of materials whichcause their conductivity to decrease when a selected analyte is puffedon the OFETs. Consequently, when an analyte is applied to the ringoscillator circuit of FIG. 10, the conductivity of OFETs P1, P3 and P5Aand OFETs N2A and N4A decreases. Therefore, each cascaded inverter isresponsive to the presence of the analyte. In addition, the output ofeach inverter (e.g., I1) is applied to the input of the next inverter(e.g., I2) along the chain with a phasal relationship that results inthe further amplification by the next inverter (e.g., I2) of the signalfrom the preceding stage (e.g., I1). For example, beginning withinverter I1, in response to an odor signal, the output of inverter I1produces a signal which is an amplified version of the response of OFETP1. Since the conductivity of P1 decreases, in response to the odor, theeffective impedance of P1 increases and the current through P1 decreasesresulting in more time being required to charge the capacitance at theoutput node of inverter I1. The output of I1 is applied to the input ofinverter I2. By making N2 an OFET whose conductivity also decreases(i.e., its effective impedance increases) in a similar manner to that ofOFET P1, inverter I2 functions to further amplify the response at theoutput of I1. This is evident from noting that as the effectiveimpedance of OFET N2 increases it causes the voltage at the output of I2to be discharged more slowly and hence the output of I2 to decrease moreslowly from its high state. Concurrently, the decrease in the voltage atthe output of I1 applied to the gate of N2 also causes N2 to conductless. Hence, the condition at the output of I2 is reinforced by thesignal at the output of I1. In a similar manner to that just described,making P3 an OFET and N3 a regular FET ensures that the signals from theprevious stages is amplified in phase with the signal generated by P3 inresponse to its sensing an analyte. This same amplification of thesensed signal within a stage in cascade with the amplified signals ofthe previous stages occurs in inverter I4.

[0061] Different forms of the cascaded inverting stages using OFETs tosense odors of the type shown in FIGS. 8 and 10 may be used to practicethe invention. An embodiment shown in FIG. 10A includes a firstinverting stage comprising a Ptype OFET, T1, and an odor-insensitiveFET, T2. The source electrode of T1 is connected to power terminal 81 towhich is applied VDD volts and its drain electrode is connected to node101. A bias voltage VB is applied to the gate of T1 to bias T1 at adesired operating point. T2 is shown as an N-type FET, but it may be aP-type FET or any load device. A control voltage, VC1, is applied to thegate of T2 to control the conductivity of T2 independently of the biasvoltage applied to T1. The drains of T1 and T2 are connected in commonat node 101 at which is produced the output of the first invertingstage. The source of T2 is returned to ground potential. The secondinverting stage includes a P-type OFET, T3, having its gate electrodeconnected to node 101, its source electrode connected to node 81 and itsdrain connected to node 103. A load, shown as a resistor RL, but which,in practice, may be a passive or active load, is connected between node103 and ground potential. The signal generated at node 103 may besupplied to any suitable signal amplifying or processing circuit.

[0062] Another embodiment is shown in FIG. 10B. the first invertingstage is similar to that of FIG. 10A. However, the second invertingstage includes an N-type OFET T3A connected at its gate electrode tonode 101, at its source electrode to ground potential and at its drainelectrode to output node 103. A fourth transistor T4 has itssource-to-drain path connected between terminal 81 and node 103. thegate electrode of T4 is shown connected to a control voltage source VC2designed to control the conductivity (impedance) of T4. T4 may be anactive load (e.g., an N-FET, a P-FET, or an OFET) or it maybe replacedby a passive resistive load.

[0063] In FIGS. 10A and 10B, the inverting stages are cascaded toenhance signal amplification and increase the sensitivity of the odorsensitive transistors to the application of analytes (odors). Thecircuits of FIGS. 8, 10, 10A and 10B illustrate the use of multiplesensors (two, or more OFETs) that are connected in circuit to coherentlyamplify the effects of an analyte by acting synchronously. Thus, smallchanges produced in a single stage in response to a weak analyteconcentration applied to the circuit are amplified over several stagesleading to an improvement in signal to noise.

[0064] Referring to FIG. 11, there is shown an oscillator 110 coupled toa counter circuit 111 whose outputs are coupled to an alarm circuit 113.The oscillator 110 may be any suitable oscillator using at least oneodor-sensitive OFET for varying the frequency of oscillation in responseto a selected odor. By way of example, oscillator 110 may be a ringoscillator as shown in FIGS. 8 and 10. Any suitable output signal of theoscillator can be applied to a counter 111 that tracks and calculatesfrequency of oscillations. If the frequency decreases below apredetermined level, for the condition where the response of the OFET toa selected odor causes the oscillator to decrease, the output of thecounter 111 activates processing circuitry 113 and activates an alarm.Alternatively, if the frequency increases above a predetermined level,for a condition when the response of the OFET to a selected odor causesthe oscillation to increase., the output of the counter 111 activatesprocessing circuitry 113 and activates an alarm.

[0065] OFETS for use in circuits embodying the invention may be formedas shown in FIG. 12. Note that a substrate 120 of standard Sielectronics (both FET and bipolar) fabricated in a conventional mannerknown in the art as integrated circuit (IC) fabrication may be used.After the fabrication of the different levels of metallization neededfor the Si circuitry, organic transistor sensor circuits are fabricatedemploying an upside-down approach. In the upside down approach, OFETcircuits are formed by sequentially defining the interconnects, the gatemetal level, a dielectric layer, source-drain metal level and theorganic semiconductor layer. The organic semiconductor sees minimalpost-deposition processing. The approach of FIG. 12 is different fromtypical circuit fabrication procedures where the transistor devices arefirst formed followed by the interconnections. However, any suitablefabrication scheme may be used to form circuits embodying the invention.

[0066] In FIG. 12, the fabrication of the organic FET circuits beginswith the deposition (above the silicon circuitry) of a thick layer (122,124) of SiO₂(for isolation). The metal lines and vias (through-holes forthe metal interconnects) are defined by photolithography and standardsemiconductor processing techniques. The interconnection metal (AI)level is defined immediately above the substrate followed by the gatemetal level (AI) and the source/drain metal level. The gate dielectric124 may be a bilayer consisting of 20 nm of Si₃N₄ and 10 nm of SiO₂,with a capacitance of 200 nF/cm². The interlayer isolation dielectrics122 are SiO₂ or Si₃N₄. The organic semiconductors are deposited as athin layer above and between the source (S) and drain (D) contacts. TheS/D contacts are coated with a gold layer by electroless/immersiontechniques to facilitate good ohmic contact with the organicsemiconductors. The underlying Si electronics and the above-lyingorganic circuitry are electrically interconnected as required throughdielectrics by forming vias. The organic transistor circuitry mayinclude any combination of the circuits described above. The activeorganic semiconductor material is deposited over the pre-formedsource-drain contacts and gate insulator may be any suitable materialfor the desired sensor selectivity.

[0067] The molecular structures of some materials used to form OFETs areshown in FIG. 13. Exemplary materials for active semiconductor layers ofP-type OFETs include:

[0068] a. didodecyl α-sexithiophene (DDα6T);

[0069] b. dioctadecyl α-sexithiophene;

[0070] c. copperphthallocyanine;

[0071] d. α-sexithiophene;

[0072] e. α,ω-dihexylsexithiophene;

[0073] f. poly(3-alkythiophene);

[0074] g. poly(3-hexylthiophene); and

[0075] h. poly(3-dodecylthiophene).

[0076] Exemplary materials for active semiconductor layers of N-typeOFETs include:

[0077] a. hexadecafluorocopperphthallocyanine (F₁₆CuPc); and

[0078] b. naphthalenetetracarboxylic diimide compounds.

[0079] These materials are listed by way of example only and any othersuitable materials may be used.

[0080] The molecular structures of some odors used to test circuitsembodying the invention are shown in FIG. 14. However, it should beunderstood that any gas, chemical vapor, odor or analyte which causes achange in the conductivity of an OFET may be sensed by circuitsembodying the invention.

[0081] The various embodiments shown herein are for purpose ofillustration, and the invention may be practiced using any suitablecircuit.

What is claimed is:
 1. An analyte sensor comprising: a series of Ninverters, where N is an odd integer greater than 1, each inverterhaving a signal input terminal and a signal output terminal andincluding a first transistor with source, drain and gate electrodes, thegate electrode being connected to the signal input terminal of thecorresponding inverter and the drain electrode being connected to thesignal output terminal of the corresponding inverter; wherein at leastone of the first transistors is an organic transistor having aconduction path whose conductivity is responsive to presence of an odorwith the signal output terminal of each inverter being connected to thesignal input terminal of the next inverter in the series.
 2. The sensorof claim 1, wherein the signal output terminal of the last inverter ofthe series and the signal input terminal of the first inverter of theseries are connected to form a ring oscillator.
 3. An analyte sensor asclaimed in claim 2, wherein each one of said first transistors is anorganic field effect transistor (OFET) having a conduction path whoseconductivity changes in response to the selected odor incident on saidfirst transistors.
 4. An odor sensor as claimed in claim 3, wherein thefirst transistors of every other inverter are of a first conductivitytype and the first transistors of the remaining inverters are of asecond conductivity type complementary to said first conductivity type.5. An analyte sensor as claimed in claim 2, wherein each one of said Ninverters includes a second transistor; wherein one of said first andsecond transistors of each inverter is an odor sensitive organictransistor and the other one of said first and second transistors isodor insensitive.
 6. An analyte sensor as claimed in claim 2, whereineach one of said N inverters includes a second transistor; wherein oneof said first and second transistors of each inverter is an odorsensitive organic field effect transistor (OFET) and the other one ofsaid first and second transistors is odor insensitive field effecttransistor (FET).
 7. An analyte sensor as claimed in claim 2, whereineach one of said N inverters includes a second transistor; wherein oneof said first and second transistors of each inverter is an odorsensitive organic transistor and the other one of said first and secondtransistors is an odor insensitive organic transistor.
 8. An analytesensor as claimed in claim 3, wherein each one of said odor sensitiveorganic transistor is a field effect transistor formed from one of thefollowing materials: (a) didodecyl α-sexithiophene (DDα6T); (b)dioctadecyl (α-sexithiophene; (c) copperphthallocyanine; (d)α-sexithiophene; (e) α,ω-dihexylsexithiophene; (f)poly(3-alkythiophene); (g) poly(3-hexylthiophene); (h)poly(3-dodecylthiophene); (i) hexadecafluorocopperphthallocyanine(F₁₆CuPc); and (j) naphthalenetetracarboxylic diimide compounds.
 9. Theanalyte sensor as claimed in claim 5, wherein each one of said first andsecond OFETs has a conductivity that decreases when the analyte OFET issubjected to the presence of the analyte.
 10. The sensor of claim 2,wherein the frequency of oscillation of the ring oscillator decreaseswhen the ring oscillator is subjected to the analyte.
 11. A gas sensorcomprising: N inverters, where N is an odd integer greater than 1,wherein each one of said inverters has a signal input terminal and asignal output terminal; each inverter having a first transistor of firstconductivity and a second transistor of a complementary conductivitytype; the first and second transistors of each inverter beinginterconnected to form an inverter, one of the first and secondtransistors of one of the inverters being an organic transistor which isresponsive to an analyte; and the N inverters being connected in acascade, with the signal output terminal of the last inverter of thecascade being connected to the signal input terminal of the firstinverter of the cascade to form a ring oscillator.
 12. A gas sensor asclaimed in claim 11, wherein said transistors are field effecttransistors (FETs) and wherein each organic transistor is an organicfield effect transistor (OFET).
 13. The sensor of claim 11, wherein thefirst and second transistors are organic field effect transistors withdifferent sensitivities to the analyte.
 14. The sensor of claim 11,wherein the analyte is one of an odor, a chemical and a vapor.
 15. A gassensor as claimed in claim 13, wherein the first transistors have activechannels formed from one of the following materials: a. didodecylα-sexithiophene (DDα6T); b. dioctadecyl α-sexithiophene c.copperphthallocyanine; d. α-sexithiophene; e. α,ω-dihexylsexithiophene;f. poly(3-alkythiophene); g. poly(3-hexylthiophene); and h.poly(3-dodecylthiophene); and wherein the second transistors have activechannels formed from one of the following materials:hexadecafluorocopperphthallocyanine (F₁₆CuPc); andnaphthalenetetracarboxylic diimide compounds.
 16. An apparatuscomprising: an odor-sensitive organic transistor having a semiconductorchannel exposed to ambient odors; and an odor-insensitive organictransistor having a semiconductor channel protected from ambient odors,the two transistors being interconnected together to generate an outputsignal dependent on currents flowing through the two transistors and onwhether an ambient odor is present.
 17. An apparatus as claimed in claim16 wherein the two transistors are of complementary conductivity typeand are interconnected to form a complementary inverter.
 18. Anapparatus as claimed in claim 17 wherein the output signal is generatedby the inverter and the inverter has a switching point dependent on bothtransistors and on whether the odor is present.
 19. An apparatus asclaimed in claim 16 further including a differential amplifier havingfirst and second inputs coupled to the odor-sensitive and odorinsensitive organic transistors, respectively, the output signal being asignal generated by the amplifier.
 20. An apparatus as claimed in claim19, further comprising; a feedback circuit to generate a feedback signalthat stabilizes an output signal of the odor-sensitive organictransistor for drift with respect to an output signal of the odorinsensitive transistor in response to a signal generated by theamplifier.
 21. A gas sensor comprising: a ring oscillator including atleast one organic field effect transistor (OFET), the ring oscillatorhaving frequency of oscillation that is a function of whether at leastone of selected odors is present; and a circuit responsive to thefrequency of oscillation of the ring oscillator and configured toprovide an indication of when the oscillations of the oscillator arewithin or outside a predetermined range.
 22. The sensor of claim 21,wherein the oscillator is a cascaded chain of inverters, the invertersincluding odor-sensitive organic transistors.
 23. An odor sensingcircuit including: an organic field effect transistor (OFET) havingsource and drain electrodes defining the ends of a conduction path and agate electrode; the conductivity of the conduction path of said OFETchanging in response to selected odors incident on the OFET and inresponse to the value of voltages applied to the gate electrode;circuitry for biasing the OFET so it is responsive to the application ofodors; and circuitry for applying an alternating signal to the gate ofthe OFET for enhancing its recovery to the condition existing prior tothe application of any odors.
 24. An analyte sensor comprising: firstand second power terminals for the application of an operating potentialtherebetween; first and second organic field effect transistors (OFETs)responsive to the presence of an analyte, each one of said OFETs havingsource, drain and gate electrodes; the source and drain of the firstOFET being connected between said first power terminal and a first node;the gate electrode of the second OFET being connected to the first node;and the source and drain of the second OFET being connected between asecond node for producing thereat a signal indicative of the presencesaid analyte and one of said first and second power terminals.
 25. Ananalyte sensor as claimed in claim 24, wherein said first and secondOFETs are of the same conductivity type and wherein the source of thesecond OFET is connected to said first power terminal.
 26. An analytesensor as claimed in claim 25 wherein a bias voltage is applied to saidfirst OFET.
 27. An analyte sensor as claimed in claim 26, wherein athird transistor is connected between said first node and the secondpower terminal, and wherein an output load is connected to the secondnode.
 28. An analyte sensor as claimed in claim 24, wherein said firstand second OFETs are of complementary conductivity type and wherein thesource of the second OFET is connected to said second power terminal.29. An analyte sensor as claimed in claim 28, wherein a bias voltage isapplied to said first OFET; and wherein a third transistor is connectedbetween said first node and said second power terminal and a fourthtransistor is connected between the second node and the first powerterminal.